Valve actuators are commonly employed to continually adjust the position of a main valve element (e.g., a butterfly valve plate) disposed within a main flow conduit and thereby control pressurized fluid flow therethrough. A generalized valve actuator may comprise, for example, a piston slidably mounted within a valve housing. A valve linkage mechanically couples the piston to the valve element, and an actuator spring biases the piston toward a retracted position. An actuator chamber is provided in the valve housing and fluidly coupled to a variable-pressure fluid source (e.g., a section of the main flow conduit located downstream of the valve element). As the fluid pressure within the main flow conduit increases, so to does the fluid pressure within the actuator chamber. The fluid within the actuator chamber acts on an exposed area of the piston in opposition to the bias force of the piston spring. When the pressure of the fluid within the actuation chamber exceeds a minimum actuation pressure, the piston moves toward an extended position and the main valve element closes.
In valve actuators of the type described above, it is commonly desirable that the valve actuator be configured such that the minimum actuation pressure is relatively high (e.g., approximately 500 pounds per square inch or more). The minimum actuation pressure is generally determined by the exposed area of the piston and the preload of the piston spring. At the same time, it may be desirable that piston movement occur over a relatively narrow pressure range in a controlled and accurate manner. The valve actuator's piston position-to-pressure characteristic is largely determined by the exposed area of the piston and by the spring rate of the piston spring. Unfortunately, in a conventional valve actuator, the actuator spring may not be capable of providing both the desired spring rate and the preload required to achieve a significant minimum actuation pressure.
Considering the above, it should be appreciated that it would be desirable to provide a valve actuator that may be calibrated to a relatively high minimum actuation pressure and that is capable of providing a significant piston displacement over a relatively narrow pressure range. Preferably, such a valve actuator would be relatively compact and lightweight and, consequently, well-suited for use aboard aircraft. In addition, it would be desirable to provide a throttle valve assembly employing such a compact and lightweight valve actuator. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended claims, taken in conjunction with the accompanying drawings and this Background.